The Periodicity of Chemical Elements
A pattern of repeating order is called periodicity. In the mid-1800s, Dmitri Mendeleev, a Russian chemist, noticed a repeating pattern of chemical properties in elements.
Mendeleev arranged the elements in order of increasing atomic mass, to form something that resembles the modern periodic table. He was even able to predict the properties of some of the then-unknown elements. Later, the elements were rearranged in order of increasing atomic number.
The periodic table enables chemists and chemistry students to:
Learn the properties of families of elements (instead of learning the properties of each element), thus saving a lot of time and effort.
Find the relationships among elements and figure out the formulas of many different compounds.
Examine the atomic numbers, mass numbers, and information about the number of valence electrons.
The elements in the following periodic table are arranged in order of increasing atomic number. The atomic number (number of protons) is located right above the element symbol.
Under the element symbol is the atomic mass, or atomic weight (sum of the protons and neutrons). Atomic mass is a weighted average of all naturally occurring isotopes.
Notice also that two rows of elements — Ce-Lu (commonly called the Lanthanides) and Th-Lr (the Actinides) — have been pulled out of the main body of the periodic table. If they were included in the main body of the periodic table, the table would be much larger.
Click here to view this table.
The periodic table is composed of horizontal rows called periods. The periods are numbered 1 through 7 on the left-hand side of the table. The vertical columns are called groups, or families. Members of these families have similar properties.
The families may be labeled at the top of the columns in one of two ways. The older method uses Roman numerals and letters. Many chemists prefer and still use this method. The newer method simply uses the numbers 1 through 18.
Chemistry Glossary
Archimedes Principle
A principle discovered by the Greek mathematician Archimedes which states that the volume of a solid is equal to the volume of water it displaces.
Chemistry Glossary
atomic number
The number of protons in the nucleus of an atom.
Chemistry Glossary
Bohr model
A model of atomic structure developed by Niels Bohr, a Danish scientist. In this model, electrons occur in orbits of differing energy levels around the nucleus of an atom.
Chemistry Glossary
condensation
The change in the physical state of matter from a gaseous state to a liquid state.
Chemistry Glossary
deposition
The change in the physical state of matter from a gaseous state to a solid state without ever becoming a liquid. The reverse of sublimation.
Chemistry Glossary
electrolytes
Substances that can conduct electricity either in the molten state or when dissolved in water.
Chemistry Glossary
electron configuration notation
A method used by chemists to represent electrons in bonding and chemical reactions.
Chemistry Glossary
electronegativity
A measure of an atom’s strength to attract a bonding pair of electrons to itself.
Chemistry Glossary
energy level diagram
A method used by chemists to diagram the electrons for an atom (including orbitals and subshells) in bonding and chemical reactions.
Chemistry Glossary
heterogeneous mixture
A mixture whose composition varies from position to position within a sample.
Chemistry Glossary
homogeneous mixture
A mixture whose composition is the same from position to position within a sample.
Chemistry Glossary
isotopes
Atoms of the same element that have varying numbers of neutrons.
Chemistry Glossary
mass number
The sum of the protons and neutrons in a particular isotope; also called atomic weight.
Chemistry Glossary
nonelectrolytes
Substances that do not conduct electricity in the molten state or when dissolved in water.
Chemistry Glossary
nuclear fission
A nuclear reaction in which an atom’s nucleus splits into smaller parts.
Chemistry Glossary
nuclear fusion
A process in which lighter nuclei of atoms join together into a heavier nucleus; essentially the opposite of nuclear fission.
Chemistry Glossary
nuclear reaction
Any reaction that involves a change in nuclear structure.
Chemistry Glossary
periodic table
A table that displays all known chemical elements in an arrangement that is based on the properties of the elements; changes over time as new elements are discovered.
Chemistry Glossary
quantum mechanical model
A model of atomic structure that is based on mathematics and can be used to explain observations made on complex atoms.
Chemistry Glossary
radioactivity
The spontaneous decay of an unstable nucleus in an atom.
Chemistry Glossary
SI system
A worldwide measurement system that is based on the older metric system. The SI comes from the French Systeme International.
Chemistry Glossary
sublimation
The change in the physical state of matter from a solid state to a gaseous state without ever becoming a liquid (such as dry ice).
Chemistry Glossary
valence electrons
The electrons in the outermost energy level of an atom, the farthest away from the nucleus.
